The present invention relates to the dehumidification of air with the aid of a moisture exchanging element, such as a heat exchanging element and dehumidifying element. An inventive moisture exchanging element is comprised of a fibre matrix that has been impregnated with waterglass. More specifically, the invention relates to a dehumidifying element that has bacteriostatic properties, and to a method of its manufacture.
It is necessary to control the humidity of the air in conjunction with certain manufacturing processes and in the storage of moisture-sensitive products. A control of this nature is also often applied in order to avoid corrosion of expensive equipment. It is normally necessary to dehumidify the air, which can be achieved with the aid of different types of air dehumidifiers. The so-called rotary sorption dehumidifier is an example of a typical air dehumidifier. Such dehumidifiers are described in SE-B-462 671, SE-B-501 507 and WO 93/08910, among other documents. FIG. 5 outlines a rotary sorption dehumidifier of this kind.
Swedish Patent Application 9804152-8, which was published after the filing date of the present application, describes an advantageous process for the manufacture of dehumidifying elements, in which paper is impregnated with a highly concentrated waterglass solution. The advantage with this process is that no energy consuming drying stage is required.
The rotor is a cylinder that has a matrix which is comprised of alternate thin pleated and planar walls that contain an hygroscopic substance, such as silica gel. The walls form in the direction of airflow narrow channels through which the air flows. Heated air that releases moisture that has fastened to the walls is conducted through a smaller sector, This air flow is then passed outside the space that shall be kept dry, via a channel system. Dry air is obtained continuously as the rotor rotates between the two air flows.
The hygiene requirements are very high in certain applications, for instance in the manufacture of pharmaceutical products and the production of foodstuffs. This will not normally constitute a problem, as it is difficult for bacteria to grow in the rotor. Most rotors have an inorganic composition and thus contain no bacteria nutrients. Furthermore, the rotor is heated at regular intervals during operation between temperatures of 100 and 140xc2x0 C., and is very dry during the whole of the cycle. Consequently, the environment is sufficiently hostile to microorganisms to satisfy hygiene requirements in operation.
However, it is difficult with conventional techniques to guarantee low bacterial growth in dehumidifying elements over long periods of inactivity. Organic material, for instance in the form of dust particles, may have fastened in the rotor and favourable conditions for microorganisms can occur locally because no heating or drying of the element takes place.
Problems can also occur when exchanging heat from air to air, including bad odours and spreading of bacteria among other things, since the moisture content of the air shall also be transferred in rotary heat exchangers (enthalpy exchange). Some of these problems may have connection with bacterial growth in the heat exchange element, which is very similar to that described above.
There is thus a need for a moisture exchange element that includes one or more components which actively counteract the growth of microorganisms.
It has now been found that a bacteriostatic moisture exchange element that solves the aforesaid problems of malodours and the growth of microorganisms can be produced by a method comprising the steps of:
a) providing paper, such as facing paper and/or fluted paper,
b) immersing the paper in a highly concentrated waterglass solution at a temperature in
the range of 45-95xc2x0 C., where said highly concentrated waterglass solution has a viscosity of at least 350 mPaxc2x7s at a temperature of 45xc2x0 C.
c) cooling the immersed paper with air at a temperature of 35xc2x0 C. at the highest, and preferably at 25xc2x0 C. at the highest;
d) producing a waterglass impregnated fibre matrix with a starting point from the paper in step c), followed by chemical conversion of the waterglass on said paper with the aid of known processes for the manufacture of a moisture exchange element; and
e) impregnating the moisture exchange element in step d) with one or more aqueous solutions of an hygroscopic salt and a water soluble substance that inhibits the growth of microorganisms.
Definitions
The term xe2x80x9cmoisture exchange elementxe2x80x9d as used in this document refers to elements that are able to reduce the moisture content of air. Examples of moisture exchange elements are heat exchange elements in rotary heat exchangers for air-to-air heat exchange, and air dehumidifying elements. An inventive moisture exchange element is comprised of a fibre matrix that has been impregnated with waterglass.
The term xe2x80x9cwaterglassxe2x80x9d, as used in this document, relates to aqueous solutions of sodium silicate (xe2x80x9csoda waterglassxe2x80x9d) or potassium silicate (xe2x80x9cpotash waterglassxe2x80x9d). Soda waterglass and potash waterglass are often designated as (Na2O)m(SiO2)n and (K2O)mSiO2)n respectively, and the mole ratio between the two oxides (n/m) can vary, as will be apparent. In the case of the present invention, soda waterglass with n/m in the range of 3.2-3.5 is preferred, and waterglass with n/m from 3.3 to 3.4 is particularly preferred.
The term xe2x80x9chighly concentrated waterglassxe2x80x9d as used in this document refers to waterglass that has a viscosity of at least 350 mPaxc2x7s at 45xc2x0 C. The upper viscosity limit is 800 mPaxc2x7s at 95xc2x0 C. The viscosity of highly concentrated waterglass at room temperature is so high as to make it extremely difficult to immerse paper in the waterglass at this temperature in practice and therewith cause the waterglass to wet the paper. Typically concentrated waterglass according to known technology has a viscosity of up to 200 mPaxc2x7s at 20xc2x0 C. Highly concentrated waterglass, on the other hand, has a much higher viscosity at 20xc2x0 C. and in its lowest concentrated form can be likened to cold syrup.
The term xe2x80x9cpaperxe2x80x9d as used in this document relates to sheets produced from organic fibres, such as cellulose, or from inorganic fibres, such as ceramic fibres, glass fibres, slag fibres, carbon fibres, mineral fibres and mixtures thereof. Inorganic fibres are preferred. It is also preferred to use glass fibres and/or mineral fibres with an admixture of up to 20% cellulose fibres or synthetic fibres. The paper will have a typical thickness of 0.1-0.3 mm. The flute height of the fluted or corrugated paper is typically 1-5 mm and its flute length is typically 1.5-7 mm. The weight of the paper is typically 20-50 g/m2.
The term xe2x80x9chygroscopic saltxe2x80x9d as used in this document refers to salts that are able to absorb air-carried water. According to the invention, the absorption capacity of hygroscopic salts shall be such that the salts will be in a dissolved state at the relative humidities in which bacteria thrive. Examples of such salts are chlorides, bromides and iodides of lithium, sodium, potassium, magnesium and calcium. Lithium chloride, calcium chloride and sodium chloride are particularly preferred.
The expression xe2x80x9cwater soluble substances capable of inhibiting the growth of microorganismsxe2x80x9d as used in this document refers to water soluble substances that have a growth inhibiting ability. Examples of such substances are azides, such as sodium azide, and water soluble silver and copper salts, such as silver nitrate, copper nitrate and copper sulphate. In principle, the invention can be practised with any water soluble substance whatsoever, provided that said substance will inhibit the growth of microorganisms.
With regard to the selection of hygroscopic salts and water soluble substances that are capable of inhibiting the growth of microorganisms, it is necessary that their use in conjunction with moisture exchanging elements will not endanger human beings and the environment. Neither should they have a negative effect on the ability of the rotor to absorb moisture. Another important factor is that costs can be kept low.
The present invention thus relates to a moisture exchange element that has been treated in a manner such as to greatly reduce the risk of growth of microorganisms, and then particularly bacteria in the moisture exchange element, even when said element is not in operational use. The moisture exchange element, e.g. a rotary sorption dehumidifier rotor, is treated by submersing or immersing the element in one or more aqueous solutions of an hygroscopic salt on the one hand, and a microorganism growth inhibiting substance on the other hand.
The inventive moisture exchange element is produced by a process that utilises highly concentrated waterglass. The waterglass is so highly viscous as to prevent paper from being impregnated therewith at room temperature in practice. However, when the highly concentrated waterglass is heated to a temperature of 45-95xc2x0 C., it becomes thinly fluid and functions as concentrated waterglass. The paper to be impregnated is thus immersed into hot highly concentrated waterglass and thereafter cooled with air that has a temperature of 35xc2x0 C. at the highest, preferably not higher than 25xc2x0 C. No drying process is required, which is a cost-saving factor. The cooled impregnated paper also has effective adhesion properties and can be readily combined into a finished moisture exchange element.
Highly concentrated waterglass can be produced, for instance, by evaporating some of the water present in the concentrated waterglass prior to manufacture.
In conventional waterglass impregnation processes, the impregnated paper is heat-dried. In conjunction with this process, the waterglass becomes thinly fluid and begins to run. There is no danger of this occurring in the inventive process, since the highly concentrated waterglass solidifies in the cooling stage of the process.
In the manufacture of the moisture exchange element, the paper that has been impregnated with waterglass is then dipped into a solution of both acid and metal salt, wherewith the composition has been chosen so that the product will obtain good mechanical strength and a high moisture exchange capacity. Examples of metal salts can be found in SE-B-462 671. Suitable acids are sulphuric acid to metal sulphates, phosphoric acid to metal phosphates, nitric acid to metal nitrates, and hydrochloric acid to metal chlorides. A metal salt solution provides a gel of good stability, although because a low pH is required to obtain good moisture absorption capacity it is necessary that a large salt surplus is present in order to be able to obtain a low pH. Although the capacity of the gel is good when solely acid is used, the gel readily breaks down in normal dehumidifying processes.
The resultant moisture exchange element is washed with water after this stage. Finally, the moisture exchange element is dipped in an aqueous solution that contains hygroscopic salt and microorganism-inhibiting substances so as to obtain a bacteriostatic moisture exchange element.
This process has many advantages. The process enables more waterglass to be used per unit of paper area, since the highly concentrated waterglass is, of course, more concentrated than concentrated waterglass. This provides the end product with a higher moisture absorption capacity. Energy consumption, and therewith production costs are lower since no drying is required after the impregnation process. Large amounts of energy are consumed when drying wet paper, as it is often necessary to use hot air or IR lamps to this end. Neither is it necessary to provide expensive drying equipment. Finally, the properties of the product are improved by virtue of the fact that there is no danger of the waterglass beginning to run after the impregnation stage.